The technology of document scanning relates to a method and an apparatus whereby the visible image(s) carried by a document is converted to an electronic image signal, for use by a variety of electronic processing means. In accordance with well known techniques, the scanning of a document operates to divide the document into an X-Y matrix comprising a large number of individual picture elements (Pixels) equal in number to X times Y. The scanner apparatus generates an electronic image signal for each document PEL.
The document's visual image is converted into an image signal by the scanning process. This image signal can be used in many ways. For example, the image signal can be transmitted to remote locations for storage or for reproduction by a printer, or the image signal can be manipulated in a manner to enhance or change the image, or the image signal can be merged with other image signals.
When a document is scanned, the entire document must be scanned to make sure that all of the visual image thereon is "read" and "stored" by the scanner. However, if an area larger than the actual document is scanned, the scanning throughput of the scanner is decreased, i.e., scanning time is wasted. Moreover, when an area larger than the actual document is scanned, scanner memory is wasted storing excess information, usually black or white space. One solution to this in the field of copier machines has been to preprogram the copier for three or four of the more common document sizes. However, this solution does not cover all possibilities and has not yet been transferred to facsimile machines or to data entry scanners for computers. Another solution that some manufacturers have developed is a scanner with discrete photoswitches along the width of the document path. This solution is not very advantageous, as it can only be used with an automatic document feed scanner and not with a stationary document scanner, since the document must pass under the vein of the photoswitch and physically trip the switch. Also, the resolution of the system would be restricted to the number of photoswitches. The photoswitches would add significantly to the cost of the scanner and reduce the overall product reliability.
Accordingly, there is a need for a scanner that can detect the exact size of the document to be scanned and only scan the exact area of the document, and thus not waste any scanning time or storage memory by scanning area outside of the actual document being scanned. It would be preferable to accomplish the above goal(s) by adding as few mechanical components to the scanner as possible, in order to keep the overall cost of the scanner down and to keep the mechanical reliability of the scanner high. There is also a need for a system to detect the presence of a document in the scan path and a system to detect whether or not the document in the scan path is crooked (page skew).